This invention relates to a process for desulfurization of coal, and more particularly, relates to desulfurization of coal using electromagnetic energy and alkali metal or alkaline earth compounds.
Carbonaceous aggregates, generally comprising coal, is a plentiful energy resource in the United States and its increased utilization is vital to meeting the nation's present and future energy needs. Despite these vast coal reserves, troublesome problems arise from an environmental standpoint in using much of this coal because of its high sulfur content. When burned, the sulfur contained in the coal is released into the atmosphere as sulfur dioxide.
Current government regulations limit sulfur dioxide emissions for coal combustion to 1.2 pounds of SO.sub.2 for each 1,000,000 BTU of energy produced. For a 12,000 BTU/pound coal, the above standard limits the total sulfur content to a maximum of about 0.7 weight percent. Chemical analyses on coal deposits from different geographic regions across the United States indicate that this pollutant level limitation eliminates direct utilization of most Eastern and Midwestern coals, which represent up to 40 percent of the total United States reserves.
Sulfur does not exist in coal in its elemental form; instead it exists in chemical combination with (a) the organic compounds in the coal, (b) iron as pyrite (FeS.sub.2) and/or (c) calcium or iron as a sulfate. These are respectively referred to as "organic," "pyritic" and "sulfate" sulfur. The sulfate sulfur is usually present in quantities less than 0.05% and is therefore not an important factor.
Organic sulfur in coal is chemically bound to the organic structure. Typical compounds involved are assumed to be the disulfides, thiophene rings, thioethers, disulfides, and mercaptans. Organic sulfur has been shown to be the most difficult type of sulfur to remove from coal. Pyritic sulfur is present in the form of a dispersion of particles which have the chemical composition FeS.sub.2. The size and form of these particles varies greatly from seam to seam and even within a given chunk.
In order to meet existing government regulations on sulfur dioxide emission, it is necessary that both pyritic and organic sulfur be removed from coal. Several prior art methods have been proposed for removing sulfur from coal compounds. These range from physical processes for separation of pyritic sulfur compounds to removal of both pyritic and organic sulfur using high pressure/high temperature chemical processes which employ chemical solvents and long treatment times. However, in general, such prior art methods have proven to be too expensive and inefficient or fail to remove both pyritic and organic sulfur in sufficient quantities to permit the treated coal to be burned consistent with current government regulations.
Prior art methods for removal of pyritic sulfur include gravity and magnetic separation techniques as well as chemical treatment techniques such as the Meyers process which treats coal with a ferric sulfate solution in an autoclave at elevated temperature and pressure for a duration in excess of ten hours to produce elemental sulfur and iron sulfate which are then removed from the coal.
Other prior art methods for removal of organic sulfur include (1) treating the coal at room temperature with a 3% solution of hydrogen peroxide, (2) reacting the coal with pressurized hydrogen and a solvent of anthracene oil at high temperature (400.degree. C.), and (3) reacting the coal at high pressure (200 atm) and temperature (352.degree. C.) with sodium hydroxide for a duration of thirty minutes.
Another prior art method for coal desulfurization is disclosed in U.S. Pat. No. 4,076,607 issued Feb. 28, 1978, by Peter D. Zavitsanos et al and assigned to the assignee of the present invention. In the process described in the Zavitsanos et al application, crushed coal is irradiated with microwave energy to induce thermochemical, in-situ, reactions to liberate sulfur in the form of stable gaseous species such as H.sub.2 S, COS and SO.sub.2. This process has shown to be effective in removing up to 50% of sulfur from coal samples containing high levels of pyritic sulfur. However, this process is significantly less effective in desulfurization of coal having a high organic sulfur content. The present invention improves on the method disclosed in the aforementioned Zavitsanos application by permitting the removal of larger amounts of sulfur from the coal compounds and in particular in removing larger amounts of organic sulfur.